There is a desire, particularly in the field of aviation, to be able to replace hydraulic controls by electromechanical controls.
In particular, proposals have already been made to use electromechanical actuators of the type having a screw-and-nut system for the purpose of controlling the movements of aircraft control surfaces or indeed for extending and retracting landing gear.
Naturally, such actuators must be particularly reliable.
That is why electromechanical actuators are generally mounted in pairs, with one of the actuators in a pair being a passive actuator capable of taking over from the other actuator in the event of it failing.
Nevertheless, the passive actuator is subjected to the same mechanical stresses as the active actuator and it can happen, particularly for primary flight controls whose surfaces are continually loaded, that both actuators fail simultaneously.
The probability of such an accident is low. Nevertheless, to limit the consequences that could arise therefrom, it is desirable to be able to declutch from control any members that might become jammed by actuator failure. In particular, for all types of accident that can happen on an actuator, it is desirable to be able to release flight control surfaces and to prevent them becoming jammed in an end-of-stroke position.
Electromechanical actuator structures of the type having a screw-and-nut system are already known in which it is possible to decouple the screw from the motor and gear box unit which drives it so that if the unit jams, then the screw can still rotate and the controlled member is not locked in position.
In this respect, reference can be made to European patent application EP 0 555 122.
Nevertheless, the structure described in that patent application does not enable the controlled member to be released in the event of the accident being due to the nut jamming relative to the screw.